It has been disclosed that certain polyesters may be formed which exhibit melt anisotropy. Such polymers commonly are referred to as being thermotropic liquid crystalline polymers. See, for instance, (a) Polyester X7G-A Self Reinforced Thermoplastic, by W. J. Jackson, Jr., H. F. Kuhfuss, and T. F. Gray, Jr., 30th Anniversary Technical Conference, 1975 Reinforced Plastics Composites Institute, The Society of the Plastics Industry, Inc., Section 17-D, Pages 1-4; (b) Belgian Pat. Nos. 828,935 and 828,936; (c) Dutch Pat. No. 7505551; (d) West German Nos. 2520819, 2520820, 2722120, 2834535, 2834536 and 2834537; (e) Japanese Nos. 43-223; 2132-116; 3017-692; and 3021-293; (f) U.S. Pat. Nos. 3,991,013; 3,991,014; 4,057,597; 4,066,620; 4,067,852; 4,075,262; 4,083,829; 4,093,595; 4,118,372; 4,130,545; 4,130,702; 4,146,702; 4,153,779; 4,156,070; 4,159,365; 4,161,470; 4,169,933; 4,181,792; 4,183,895; 4,184,996; 4,188,476; 4,201,856; 4,219,461; 4,224,433; 4,226,970; 4,230,817; 4,232,143; 4,232,144; 4,238,598; 4,238,599; 4,238,600; 4,242,496; 4,245,082; 4,245,084; 4,247,514; 4,256,624; 4,265,802; 4,267,304; 4,269,965; 4,272,625; 4,279,803; 4,284,757; 4,285,852; 4,287,332; 4,294,955; 4,299,756; 4,311,824; 4,314,073; 4,318,841; 4,318,842; 4,330,457; 4,332,759; 4,333,907, 4,335,232; 4,337,190; 4,337,191; 4,339,375; 4,341,688; 4,346,208; 4,347,349; 4,351,917; 4,351,918; 4,355,132; 4,355,133; 4,355,134; 4,359,569; 4,360,658; 4,362,777; 4,370,466; 4,371,660; 4,374,288; 4,375,530; 4,381,389; 4,384,016; 4,393,191; and 4,394,498; (g) U.K. Application No. 2,002,404; (h) British Pat. No. 1,568,541; and (i) European Patent Application Nos. 24,499 and 45,499. Amide groups and/or carbonate groups additionally may be present in the polyesters which exhibit melt anisotropy.
The thermotropic liquid crystalline polymers of the prior art are formed by techniques whereby the requisite reactive groups which form ester-groups along the polymer chain are carefully reacted so as to provide a stoichiometric balance of reactive groups. For instance, if a relatively volatile monomer, such as hydroquinone or hydroquinone diacetate, is employed as a reactant, an excess of this monomer sometimes is provided to compensate for the quantity of this reactant which is evolved and lost by volatilization through the use of the specific polymerization conditions selected. When the various ester-forming monomers are provided and react with each other under stoichiometrically balanced conditions, a polymer is produced having the random presence of the requisite ester-forming groups at the ends of the polymer chain. These end groups unless otherwise end capped in a further reaction step have the propensity upon subsequent thermal processing to react with each other and to cause the polymer chains to continue to grow in length. The thermal processing of such polymers to increase the molecular weight in the solid state is disclosed, for example, in U.S. Pat. Nos. 3,975,487; 4,183,895; and 4,247,514. The continued polymerization via a condensation reaction results in the simultaneous evolution or off-gassing of relatively small molecular by-products and an elevation in the melt viscosity of the resulting polymer upon any subsequent melt processing. Such increase in melt viscosity may require the selection of different melt processing conditions at different points in time as the average chain length increases. For instance, it may be desirable to modify the injection molding conditions when forming molded articles from the resulting anisotropic melt-forming polyester. Accordingly, the melt processing of such polymers may require the continued or periodic monitoring of the molten polymer viscosity and the adjustment of melt processing conditions in an effort to yield uniform molded products if the molten polymer is provided at an elevated temperature for an extended period of time.
Additionally, the melt devolatilization of previously formed thermotropic liquid crystalline polymers of the prior art is desirable to remove substantial quantities of voidforming volatile materials contained therein. However, if such devolatilization is practiced for a sufficient period of time to substantially remove such volatiles, a continued molecular weight increase also will occur.
It is an object of the present invention to provide an improved process for forming a thermally stable thermotropic liquid crystalline polyester of predetermined chain length.
It is an object of the present invention to provide an improved process for forming a thermally stable thermotropic liquid crystalline polyester which is particularly suited for melt processing to form substantially uniform molded articles on a consistent basis.
It is an object of the present invention to provide an improved process for forming a thermotropic liquid crystalline polymer which will generate a significantly reduced amount of volatile by-products during further melt processing.
It is an object of the present invention to provide an improved process for forming a thermotropic liquid crystalline polymer which is capable of undergoing vigorous melt devolatilization without concomitantly producing an excessive polymer chain growth and a significant change in the melt vicosity of the resulting polymer.
It is an object of the present invention to provide a thermally stable polyester which is capable of forming a liquid crystalline melt phase without the need of employing a conventional end-capping reaction of a previously formed polymer.
These and other objects, as well as the scope, nature and utilization of the invention will be apparent to those skilled in the art from the following detailed description.